Usually, pipe for pipeline construction is coated with a mainline polymer coating leaving the ends of the pipe bare to allow the exposed ends to be welded together at a pipe joint. There are several different ways used in the art to coat pipe joints. One such way is the use of a heat shrinkable sleeve applied around the welded pipe joint. The sleeve is fitted to the pipe joint, then heat shrunk down onto the joint. Alternatively, a film or tape wrapping can be used. This can be, for example, a polypropylene film, which is wrapped around the pipe joint. The film or tape wrapping requires use of heat and tension to fuse the wrapping to itself as it is applied to the pipe joint. Typically, the film or tape wrapping is also applied over the ends of the pipeline coating, to form a complete coating of the exposed pipe. Such film or tape wrapping typically requires pre-heating of the exposed pipe to facilitate or enable fusion of the wrapping to the pipe. A further alternative way of coating the welded pipe joint is an injection moulding method, whereby the exposed pipe joint is encased with a mould, and a polymer, such as polypropylene or polyurethane, is pressure injected into the mould. The polymer is allowed to cool, and the mould is removed, leaving a pipe joint that is coated with polymer. As can be appreciated, such a method also benefits from the pre-heating of the exposed pipe, so that the injected polymer is not cooled too quickly upon contact with the pipe and a good adhesion to the substrate is affected. These injection moulding and film or tape wrapping methods have an advantage over the use of heat shrinkable sleeves in situations where the mainline coating is extremely thick, and the joint cavity needs to be filled.
In the case of heat shrinkable sleeves applied around a welded pipe joint during pipeline construction, typically, such sleeves are heated and shrunk down onto the joint or other article using a hand held flame torch (see, for example, U.S. Pat. No. 4,472,468, entitled “Heat Shrinkable Covering and Method for Applying Same”, issued Sep. 18, 1984, which is incorporated herein by reference). In some cases, this manual operation produces an imperfect installation because of air trapped underneath the shrunk down sleeve. This can arise when the ends of the sleeve are shrunk down before the middle portion of the sleeve. Artful application of the torch is critical. If the torch is tilted outwardly the end zones of the sleeve may shrink first leading to air entrapment. Windy conditions may spread the flame and shrink the end zones of the sleeve prematurely. Further, unless the torch is moved carefully, the torch flame may burn the sleeve and cause it to split. Where a large area needs to be heated, it becomes difficult or impossible to maintain the heat while the sleeve is being shrunk; this leads to wrinkling of the sleeve, imperfect installation due to trapped air, tearing, or scorching of the heat shrink material. Sometimes, it also results in improper or incomplete adherence of the heat shrink material around the welded pipe joint.
Before the field joints coatings are applied over the joint, the joint has to prepared in prescribed manner required for the coating type. Typically, for heat shrink sleeves, tapes and wrap systems, and injection molding, the steel is usually grit blasted, and in rare cases, power wire brushed to obtain white metal or near white metal finish. The mainline coating is usually prepared in order to clean it, and often impart roughness by abrading or light grit blasting. The joint usually requires preheating to remove moisture, but more importantly to achieve certain temperature consistent with coating type to obtain adhesion or fusion of the joint coating to the steel and to the mainline coating. For example, for polypropylene type shrink sleeves, where the adhesive may have melting point of around 155° C., the preheat of the steel is often 180° C. The preheating is often done by using induction heating, which heats the steel only, and indirectly the mainline polymeric coating. Since the exposed steel is directly heated, it can be taken to the desired temperature readily, however, the mainline coating gets heated via the heat conducted by the heated steel underneath. Therefore there is time lag for the coating surface to heat up, and there is often 40-100° C. temperature difference on the steel and coating surface temperature, depending on the coating thickness. For example, on a 610 mm diameter pipe with a wall thickness of 25 mm, when the joints steel temperature reaches 180° C., a polypropylene coating of a 5 mm thickness may only reach 100° C. -120° C. Therefore when subsequently applying a heat shrink sleeve over a joint with such substrate heat profile, the sleeve requires more heat to be applied near the ends overlapping over the mainline coating in order for the sleeve to adhere to the exposed steel and the mainline coating to form a sound protective seal.
The differences in materials in the exposed steel, and the mainline coating result in different heat requirements during the preheating. In some cases, for example, excess heat at the pipe joint may overheat the mainline coating and damage it. During preheating, more intense heat is generally required on the exposed steel, and less intense heat being required the coated pipe, due to the properties of the material used in the mainline coating. For example, where a pipe has a thick mainline coating, made of polymeric material, the exposed steel will have different specific heat, heat resistance, retention and conductivity characteristics than the mainline coating. Thus, the exposed steel may require a more intense heat (which would damage the mainline coating), but may require it for a shorter amount of time, with the mainline coating requiring a lower heat, for a longer period of time, in order for the heat to absorb into the coating thickness. In addition, where heat is applied using a hand held flame torch, the operator of the torch must bear in mind the differences in thicknesses of the different zones to be coated, and radially adjust the torch position accordingly. For example, the thickness of the mainline coating may be substantial, and the operator may need to move the torch a substantial distance in order to keep the same distance between the torch and the area to be heated.
The sizes and configurations of torches and heating implements vary greatly in the field, as do the sizes and configurations of the pipes to be treated. Sometimes, large powerful torches are used. These tend to flare out greatly and do not allow focused heating of the shrink sleeve, thereby causing air entrapment due to prematurely shrunk sleeve ends. Sometimes four torches are used to shrink a sleeve to get fast production rates, with two operators on one side of the pipe and two on the other. This practice makes it very difficult to selectively heat the middle portion of the sleeve before the ends, and makes it near impossible to apply even, consistent heat throughout the area to be heated, to accurately control the amount of heat applied to different areas to be shrunk, or to accurately control the order in which the various areas are to be heated. Often, in order to obtain proper adhesion of the sleeve, it is required to maintain a minimum preheat temperature of the substrate, usually steel pipe and the mainline coatings on the pipe sections adjacent to the joint. Even when fewer area is to be heated, or where less torches are employed, certain areas, for example, the mainline coating adjacent the opposite end of the pipe, tends to have cooled below the minimum preheat temperature, so that the sleeve does not bond thereto. Therefore during the shrinking operation, extra prolonged heat has to be applied to sleeve area overlapping onto the mainline coating in order raise the adhesive-mainline coating interface to sufficient temperature to achieve a sound bond. With the flame torches, this is difficult as prolonged heating can scorch and damage the sleeve, and sometimes lead to splitting. The need for the extra prolonged heating is exacerbated by the fact that during the preheating of the joint, the mainline coating surface maybe 40° C.-100° C. cooler than the adjacent steel, as described earlier. Therefore, focused prolonged heating is imperative to achieve a good bond on the overlap coating.
Similar considerations should be taken into account when pre-heating a pipe joint prior to film or tape wrapping, or injection moulding.
U.S. patent Ser. No. 13/230,258, filed Mar. 12, 2010, and incorporated therein by reference, describes an apparatus for heating an elongate tubular article, and/or for heating a heat shrinkable sleeve applied around an elongate tubular article. The apparatus comprises a frame member adapted to be disposed around said article, the frame member provided with a heater device adapted to heat the article and/or the sleeve surrounding said article. The patent also describes a controller for operating the heater device. Ser. No. 13/230,258 teaches that the heater device can comprise two or more independent heater portions adapted to heat respectively two or more distinct longitudinally spaced zones of the sleeve, and the controller is able to operate the heater portions simultaneously or sequentially, and/or at different heating intensities/wavelengths/temperatures. The heater device therein described can comprise two or more regions of different diameters, to better conform to an elongate tubular article of varied diameter.
The heating elements used in such a device are taught to be any known form of heating element, including, in certain embodiments, infrared electrical elements, such as Unitube heaters available from Casso-Solar Corporation, Pomono, N.Y., United States of America. These infrared elements may be in the form of, for example, quartz tubes or ceramic tiles. Alternatively, they may comprise diffused gas combusting devices, powered for example by propane or natural gas. Examples of these include gas catalytic heaters available from Casso-Solar and from CCI Thermal Technologies, Edmonton, Alberta, Canada. Further examples include burners comprising metallic or ceramic matrixes that diffuse the flame and then radiate the heat outwards, such as Fibergas-II™ heaters, again from Casso-Solar, and heaters using gas diffused through perforated ceramic matrices, as supplied by Infragas S.p.a., Caselle Torinese, Italy.
U.S. Ser. No. 13/230,258 teaches, in certain embodiments, the use of thin film or otherwise flexible infrared electrical elements are used (also called “foil”, or “flexible ribbon” heating elements). Examples of such elements include the V-series medium wavelength infrared panel heaters available from Casso-Solar Corporation, Pomona, N.Y., United States of America, as well as strips, sheets, planar thin foil heaters, corrugated ribbon foil, carbon loaded film, metal film photo patterned with runs of graphite material, conductive material sprayed or doctor bladed on a support medium, expanded metal, or wire resistive elements, such as sinuated wire. Stamped thin metal sheets having low mass for fast heating/cooling and minimal thermal lag, which can be attached to a high temperature insulation board having low thermal conductivity, low thermal mass and low heat capacity to minimize stored heat, are one such example. The patent teaches that thin film elements can be mounted on a high temperature insulating material and/or onto refractory insulating material in a variety of configurations, including linear, sinusoidal, or other configurations, as required or desired by the heating configuration and sequence, and that the use of such thin film or otherwise flexible heating element has numerous advantages, including: facilitating the manufacturing of the apparatus in varying shape and size, to tightly conform to the area to be heated; allowing customization of apparatus size and shape; tapering of the shape of the device to account for the difference in radius of the uncoated pipe such as the pipe proximal to the pipe joint and the radius of the mainline coating; all allowing for an improved and more even heat distribution along the various areas to be heated, and avoidance of the problems of burning or splitting of the sleeve.
U.S. Ser. No. 13/230,258 teaches that, by using stamped thin metal strips, different areas can be heated to different temperatures or at different time periods within the heat shrinking process, simply by having separate heating elements applied to different areas of the apparatus, and having each of these separate heating elements controlled individually by the controller. The different heating elements may be individually thermostatically controlled by the controller, and/or may have different heating characteristics (for example, made of different substrates or having a different coil thickness) to enable the variation in heating. However, the patent only teaches the use of such separately and individually controlled heating elements in a longitudinal orientation, for example, to shrink down the middle zone of the sleeve before the end zones, avoiding air entrapment, and permitting heating the areas of bare pipe to a higher heat level than the areas of coated pipe, thus preventing damage to the pipe coating while providing the bare pipe with optimal heat.